1. Technical Field of the Invention
The present invention relates generally to tire inflation pressure detecting apparatuses and wireless electric power charging systems. More particularly, the invention relates to a batteryless, direct-type tire inflation pressure detecting apparatus which is based on a transponder method and has an improved configuration for power saving.
2. Description of the Related Art
A conventional direct-type tire inflation pressure detecting apparatus for a vehicle generally includes at least one transmitter and a receiver.
The transmitter is directly installed to a wheel of the vehicle and includes a pressure sensor working to sense inflation pressure of a tire fitted on the wheel. The transmitter is configured to transmit a pressure signal indicative of the inflation pressure of the tire sensed by the pressure sensor.
The receiver is installed to the body of the vehicle and includes at least one antenna. The receiver is configured to receive, through the antenna, the pressure signal transmitted by the transmitter and determine the inflation pressure of the tire based on the received pressure signal.
In the above arrangement, the transmitter is conventionally supplied with electric power by a battery provided on the wheel of the vehicle. On the other hand, developments have recently been made to supply electric power to the transmitter without using a battery.
For example, U.S. Patent Application Publication No. 2005-0280523 discloses a batteryless tire inflation pressure detecting apparatus which is based on a transponder method.
According to the transponder method, the receiver (i.e., the interrogator) works to transmit, through the antenna thereof, a charge wave (i.e., a radio wave for electric power charge) to the transmitter, thereby charging a charging unit of the transmitter (i.e., the transponder). When the charging unit is completely charged with electric power, the transmitter is then activated by the charged electric power to transmit the pressure signal to the receiver.
However, in such a batteryless tire inflation pressure detecting apparatus, a charge-possible range of angular position of the transmitter, within which the level of the charge wave received by the transmitter is above a necessary level for charging the charging unit, is limited when the transmitter rotates together with the wheel.
Moreover, when the charge wave transmitted by the receiver and the pressure signal transmitted by the transmitter are at frequencies within the same frequency band, it is very difficult for the receiver to transmit the charge wave while receiving and accurately demodulating the pressure signal.
Thus, it is necessary for the transmitter to transmit the pressure signal when the receiver stops transmitting the charge wave, so that the pressure signal and the charge wave can be prevented from being simultaneously transmitted.
In view of the above, in the batteryless tire inflation pressure detecting apparatus disclosed in the above patent document, the receiver is configured to perform a “packet charge” for the charging unit of the transmitter, during which operation of the receiver is changed alternately between an ON and an OFF mode.
More specifically, referring to FIG. 11, in the ON mode, the receiver transmits the charge wave without simultaneously receiving signals. On the other hand, in the OFF mode, the receiver stops transmitting the charge wave and is allowed to receive signals. Thus, through changing the operation thereof alternately between the ON and OFF modes, the receiver intermittently transmits the charge wave, thereby charging the charging unit of the transmitter.
When the charge voltage of the charging unit has increased to a predetermined level, the transmitter starts to transmit an informing signal a plurality of times continuously and then transmit the pressure signal. Here, the informing signal is a signal which is informative of that the pressure signal will be transmitted by the transmitter immediately after the signal. On the other hand, when the informing signal is first received by the receiver in the OFF mode, the receiver keeps the operation thereof in the OFF mode until the pressure signal following the informing signal is completely received by the receiver.
Further, to allow the receiver to reliably receive the informing signal, there is defined the relationship of Ti≧(Ton+Toff), where Ti is the length of that time period for which the informing signal is continuously transmitted the plurality of times, Ton is the length of each time period for which the receiver operates in the ON mode, and Toff is the length of each time period for which the receiver operates in the OFF mode. In addition, Toff is so set as to be not shorter than a time required for the receiver to receive and identify once the informing signal transmitted by the transmitter.
With the above configuration, however, the total time required for one transmission of the pressure signal is mostly spent on charging the charging unit of the transmitter, as shown in FIG. 11. Accordingly, to shorten the total time, it is desired to reduce the electric power consumption of the transmitter. However, when Ti is shortened for the purpose of reducing the electric power consumption, it will be difficult for the receiver to reliably receive the informing signal.